Modern oil field operators demand access to a great quantity of information regarding the parameters and conditions encountered downhole. Such information typically includes characteristics of the earth formations traversed by the borehole and data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole, which commonly is referred to as “logging,” can be performed by several methods including wireline logging, tubing-conveyed logging, and “logging while drilling” (“LWD”).
In wireline logging, a sonde is lowered into the borehole after some or all of the well has been drilled. The sonde hangs at the end of a long cable or “wireline” that provides mechanical support to the sonde and also provides an electrical connection between the sonde and electrical equipment located at the surface of the well. In accordance with existing logging techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.
Tubing-conveyed logging is similar to wireline logging, but the sonde is mounted on the end of a tubing string. The rigid nature of the tubing string enables the tubing-conveyed sonde to travel where it would be difficult to send a wireline sonde, e.g., along horizontal or upwardly-inclined sections of the borehole. The tubing string can include embedded conductors in the tubing wall for transporting power and telemetry, or a wireline cable can be fed through the interior of the tubing string, or the sonde can simply store data in memory for later retrieval when the sonde returns to the surface.
In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being drilled, thereby enabling measurements of the formation while it is less affected by fluid invasion. While LWD measurements are desirable, drilling operations create an environment that is generally hostile to electronic instrumentation, telemetry, and sensor operations.
One of the instruments that has been employed in at least the LWD and wireline logging environments is a nuclear magnetic resonance (“NMR”) logging tool. NMR tools operate by using an imposed static magnetic field, B0, to preferentially polarize the nuclear spins of the formation nuclei parallel to the imposed field. The nuclei converge to their equilibrium alignment at a measurable rate. When this convergence occurs after the nuclei have been placed in a cooperative initial state (discussed below), it is known as magnetization recovery, or simply, recovery. The time constant for recovery is called the “spin-lattice” or “longitudinal” relaxation time T1.
During or after the polarization period, the tool applies a perturbing field. Usually the perturbing field takes the form of a radio frequency (“RF”) pulse whose magnetic component, B1, is perpendicular to the static field B0. This perturbing field moves the preferential orientation of the nuclei into the transverse plane. The frequency of the pulse can be chosen to target specific nuclei (e.g., hydrogen). The polarized nuclei are perturbed simultaneously and, when the perturbation ends, they precess around the static magnetic field gradually returning to alignment with the static field once again. As previously mentioned, the rate at which the nuclei recover their initial alignment is governed by the “longitudinal” relaxation time constant T1. There is a second time constant to this process which can also be measured, and that is the rate at which the precessing nuclei (which are phase-aligned by the perturbing field) lose their phase alignments with each other. The relaxation time constant of this coherence loss is the “spin-spin” or “transverse” relaxation time constant T2.
Most commonly. NMR tool measurements are obtained using an RF pulse sequence known in the art as the Carr-Purcell-Meiboom-Gill (“CPMG”) pulse sequence, and measuring the detectable “echo” signals generated by the precessing nuclei. The CPMG pulse sequence is most frequently used for measuring T2 distributions, but a popular method for measuring T1 distribution operates by observing the effect of different recovery time spacings between CPMG experiments. Other NMR tool methods employ consecutively spaced RF perturbations followed by a CPMG sequence to probe the magnetization build up. As is well known in the industry, either the T2 or T1 relaxation time distribution information can be readily converted into measurements of porosity (i.e., the volume fraction of void space in the formation), hydrocarbon saturation (i.e., the relative percentage of hydrocarbons and water in the formation fluid), and permeability (i.e., the ability of formation fluid to flow from the formation into the well bore). For a more comprehensive overview of the NMR technology including logging methods and various tool designs, the interested reader is directed, for example, to the book by Coates et al. entitled “NMR Logging: Principles and Applications” distributed by Gulf Publishing Company (2000), and hereby incorporated herein by reference for background. Additional description of NMR logging techniques is provided, for example, in U.S. Pat. Nos. 4,710,713; 4,717,876; 4,717,877; 4,939,648; 5,055,787; 5,280,243; 5,309,098; 5,517,115, 5,557,200; 5,696,448; 5,936,405; 6,005,389; 6,023,164; 6,107,796; 6,111,408; 6,242,913; 6,255,819; 6,512,371; 6,525,534; 6,541,969; 6,577,125; 6,583,621, 6,646,437; 6,717,404; and 7,463,027 which are hereby incorporated herein by reference.
Traditionally, NMR tools have a relatively large sensing region but also suffer from significant energy losses when employed in large boreholes with conductive fluids. The relatively large distance between the tool and the sensing region essentially offsets whatever gains the tool achieves through the use of a larger sensing volume, necessitating that the tool be custom designed for use in a relatively small range of borehole sizes.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure, but on the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed with the given embodiments by the scope of the appended claims